my question is, steam say game opens on Dec 9th and other platforms are saying the 10th. which is right?
Both. It's a global release which means it releases the exact same second everywhere. So some parts of earth will be on Dec 9 and some on Dec 10 at that moment. I've seen a few different times listed around the interwebs, but my best guess is it will be something like:
Tokyo - 9 am Dec 10, 2020
Moscow - 3 am Dec 10, 2020
Warsaw - 1 am Dec 10, 2020
London - Midnight Dec 10, 2020
New York - 7 pm Dec 9, 2020
Los Angeles - 4 pm Dec 9, 2020